Why are the SDA drivers where they are? Ken S maybe?

madmax

While talking to Hoosier we realized the SDA drivers are different distances away from the active driver on different models of SDA's. For example, on the 1C's the SDA drivers are right next to the active drivers (and the tweeters above) but on most other SDA's the drivers are maybe 7" center to center with the tweeters in between them. Maybe it's just to fit the tweeters in there but I'm still wondering if one way is optimum.
madmax